Catalytic cracking involves the processing of gas oils using catalysts to crack the carbon-carbon bonds. In particular, catalytic cracking consists of breaking saturated C12+molecules into C2-C4 olefins and paraffins, gasoline, light oil, and coke. Cracking serves to lower the average molecular weight and to produce higher yields of fuel products. The majority of the reactions are endothermic and heat must be supplied to the cracking process. Cracking can be either purely thermal or thermal and catalytic. In general, it is desirable to promote catalytic cracking over thermal cracking since thermal cracking produces unwanted by-products.
The FIGURE is a diagram of a typical fluidized catalytic cracking (FCC) unit 10. In particular, these units include a riser reactor 16, which acts as a plug flow reactor where catalytic cracking occurs at operating temperatures of about 950-1000° F.; and a catalyst regenerator 14 which serves to remove the excess carbon laid down on the catalyst as coke that is produced by the cracking reactions. In the riser reactor 16, hot regenerated catalyst 18 from the catalyst regenerator is diluted with steam 19 and a preheated feed composition (typically at 300° F. or greater) 20 is injected through a spray nozzle 21 just above the bottom of the riser reactor. Catalyst flow is controlled by valves and changing the density in the standpipe 23 with steam 19. Regenerated catalyst 18 flows down through standpipe 23 from the regenerator to be lifted to the reactor 16 by steam 19 and fresh feed 20. The dilute phase of the catalyst 22 flows up the riser at temperatures of about 750° F. and discharges the hot reactants into the upper part of the riser reactor 16. Reacted hydrocarbon vapors are then separated from the dense phase of the spent catalyst 24. In particular, the reacted hydrocarbon vapors are purified by passing through cyclone separators 12 to reduce particulate content and the separated vapors, which constitute the catalytic products 25, are sent to a fractionator. The catalyst with coked surface drops to the regenerator 14 where it is present as a dilute phase 26. In the regenerator 14, the coke is burnt off at temperatures of about 1200°-1300° F., and a dense phase of regenerated catalyst 18 is returned for another reaction pass.
It is known that feed atomization in the base of the FCC riser reactor is a problem in hydrocarbon processing. In particular, it is difficult to contact many tons per hour of hot, regenerated cracking catalyst with large volumes of heavy oil feed, while ensuring the complete vaporization of the feeds at the bottom of the riser reactor. Part of this problem can be attributed to the use of heavier feeds in FCC units. In particular, heavier feeds are more difficult to vaporize because of their high boiling points, and the heavy feeds are harder to atomize because of their high viscosity, even at the high temperatures which exist in FCC riser reactors.
Effective operation of several process units in hydrocarbon processing depend on the ability to atomize the hydrocarbon stream. The preferred reaction in a catalytic cracker occurs within the pores of the catalyst. This requires vaporization of the feed. At a fixed reactor temperature, the kinetics of vaporization are largely determined by the size of droplets introduced into the reactor. In particular, for a fluid catalytic cracker, a fluidized bed of catalyst is sprayed with hydrocarbon at the bottom of the riser reactor. The creation of small hydrocarbon droplets in the spray is a key contributor to unit efficiency as it promotes catalytic cracking over thermal cracking. A feed injection system should provide both rapid vaporization and intimate contact between the oil and catalyst. Rapid vaporization requires atomization of the feedstock into small droplets with narrow size distribution.
Efficient atomization for these hydrocarbon processes has been the focus of numerous mechanical process changes. For example, the mechanical improvements include refinements such as inclusion of internal barriers in the fluid catalytic cracker, impingement blocks and improved methods of spray blast. All of these approaches rely on enhancing various factors known to be important in spray atomization. Another approach has been to introduce an alternate mechanism of atomization. Generally, this is referred to as secondary atomization. Primary atomization relies on the balance between the cohesive nature of the fluid being sprayed and the aerodynamic forces impinging on a drop that drives breakup. However, in secondary atomization a second factor is introduced that induces droplet breakup.
Secondary atomization as a means of improving combustion processes is well established. For example, U.S. Pat. No. 3,672,853 describes a process for the preparation of a liquid fuel suitable to be handled in a pressure-type atomizer, using a hydrocarbon-containing feed as base material, in which process a gas is dissolved in the feed and improves atomization of the fuel. As the result of the pressure in the pressure-type atomizer decreasing very rapidly, the solubility of the gas also decreases. Gas thus being liberated contributes to the liquid droplets being split up to a larger extent.
U.S. Pat. No. 6,368,367 discloses an aqueous diesel fuel composition for internal combustion engines that includes a continuous phase of diesel fuel; a discontinuous aqueous phase that is comprised of aqueous droplets having a mean diameter of 1.0 micron or less; and an emulsifying amount of an emulsifier composition including an ionic or non-ionic compound having a hydrophilic lipophilic balance (HLB) in the range of about 1 to about 10.
Whereas secondary atomization as a means of improving combustion processes is well established, there has been little, if any, effective transfer of this technology to the hydrocarbon process field.
An article in Oil and Gas Journal, Mar. 30, 1991, pp 90-107 describes a means of mixing steam to the feed of a fluid catalytic cracker by feeding an emulsified fuel that separates into a two-phase (i.e. water vapor and liquid oil) flow prior to the spray nozzle at the bottom of the riser reactor. This two-phase approach provides for extra energy of mixing, meaning that the oil and catalyst mix faster, providing less opportunity for the oil to thermally crack. However, this two-phase approach does not affect the transport properties of the hydrocarbon feed. Moreover, because it is a two-phase flow on the feed side of the spray nozzle, there is no phase change across the nozzle to increase atomization efficiency.
An article in Petroleum Refinery Engineering, vol. 31 (11) pp. 19-21, 2001 discloses the use of surfactants to stabilize a water-in-oil emulsion. In particular, a feedstock for heavy oil catalytic cracking is disclosed as being emulsified and formed into a stable water-in-oil emulsion by a non-ion surfactant compound. The water is dispersed uniformly in oil with drops of about 5 microns. In particular, the emulsified feedstock is first atomized by pumping through an atomization nozzle. After subsequently being in contact with high temperature catalyst, the water drops rapidly vaporize, causing the effect of secondary atomization whereby the oil drops break into smaller drops, which are easier to get into the reaction channel of the catalyst. The yield of light oil is reported to have been enhanced and the yields of dry gas and coke decreased, whereas product qualities of diesel and gasoline remain unchanged. The nature of the surfactant is not disclosed, except that it is a blend of materials with an HLB of 5.8. According to data obtained from surfactant formulatory indices, surfactants with HLB's in this range are reported to stabilize water-in-oil emulsions. The emulsified feedstock in this reference was tested in a pilot plant, under operating conditions very different than those encountered in working plants. For example, the reference discloses the use of emulsified feedstock temperatures of about 85-90° C. Under the relevant temperature and pressure conditions encountered at working hydrocarbon processing plants, non-ionic surfactants with an HLB of 5.8 do not stabilize water-in-oil emulsions, as discovered by the present inventors.
It would be advantageous, therefore, to provide a feedstock composition for use in hydrocarbon process units, where a water-in-oil emulsion of small droplet size could be formed and stabilized under conditions typically encountered under process (or modified process) conditions. In particular, it would be advantageous to provide a water-in-oil emulsion with improved atomization properties that would be stable under the conditions relevant for FCC systems. Such conditions would include elevated temperature (greater than 300° F.) and elevated pressure conditions (pressure greater than steam vapor pressure) at the working temperature.